1. Field of the Invention
The present invention relates to electromagnetic flowmeters used to monitor fluid flow.
2. Description of Prior Art
Electromagnetic flowmeters operate, as is well known, by passing a current through a coil to generate a magnetic field and detecting an electro-motive force, measured in volts, induced in a fluid passing through the field. The output voltage, E, is proportional to the product of the field, B, and flow velocity, v, and thus: EQU E=k.multidot.B.multidot.v. (equation 1)
The constant k will depend on factors such as the sensitivity of the meter and the spacing of the electrodes, but can be considered constant for a given apparatus. It is generally assumed that the magnetic field strength in the coil is linearly proportional to the current, I, flowing in the coil so that: EQU B=m.multidot.I (equation 2)
The value of m is normally considered a constant approximating to: ##EQU1##
Thus, the output signal is proportional to the product of current and velocity, and can be expressed by substitution for B from equation (2) into equation (1) as: EQU E=km.multidot.I.multidot.v. (equation 3)
Measurement is normally carried out at a fixed current, provided by a constant current source, but, in order to take into account small fluctuations in the current, the output current is usually measured, and the velocity is determined from the ratio of output voltage to input current from equation (3) rewritten as: EQU v=(1/km).multidot.(E/I) (equation 4)
The constant (1/km) thus relates the flow velocity to the ratio of measured voltage to applied coil current, which we will hereafter refer to as R, giving v=R.multidot.E/I.
The above equation represents a reasonably good approximation for most purposes, at constant current. However, the inventor has appreciated that, in fact, the field strength is not directly proportional to the input current. Thus, errors can arise, which can be significant if the flowmeter is highly accurate (the inventor has found the problem particularly acute in the development of meters of accuracy of the order of 0.1%), hitherto necessitating the use of high stability accurately calibrated constant current sources, which can add to manufacturing time and cost, and may present particular problems in battery powered equipment. In addition, pursuant to the invention, to optimise power consumption, it has been proposed to make measurements at one of a plurality of currents or at a current which may be continuously varied.